Certain vehicles, such as loaders and scrapers, have two wheeled body members connected by an upright joint. Such vehicles are most efficiently steered by causing relative pivotal movement of the two body members about the joint; and conveniently such relative pivotal movement is provided by a hydraulic steering system under the control of a steering wheel on one of the body members. The system includes a pair of hydraulic jacks coupled between the members on opposite sides of the joint, and steering is accomplished by extending one jack while simultaneously retracting the other jack. The hydraulic system commonly includes a constant displacement pump driven by the vehicle engine, a three-position control valve and a control element operable by rotation of the steering wheel for moving the control valve between a neutral position and two offset positions for simultaneously extending one jack and retracting the other as heretofore described.
In accordance with U.S. Pat. No. 3,444,948, such hydraulic steering systems may be provided with a follow-up means which permits them to be steered by conventional manipulation of the steering wheel. The follow-up means disclosed and claimed in U.S. Pat. No. 3,444,948 utilizes a master cylinder coupled between the wheeled members for detecting pivoting at the joint, together with a slave cylinder which is hydraulically connected to the master cylinder for synchronized movement therewith, and a coupling between the slave cylinder and the valve control element to move said element in a direction opposite to that produced by turning of the steering wheel. With the follow-up system of U.S. Pat. No. 3,444,949, when the operator has turned the steering wheel a certain distance and then stopped turning the wheel, the follow-up system acts to return the three-position control valve to its neutral position so that the turning radius remains constant until the operator rotates the steering wheel farther in the same direction or else rotates it in the opposite direction.
Even with the above described follow-up system, there is a problem in the steering of vehicles of the herein described type. The constant displacement hydraulic pump is driven directly by the vehicle engine, so that pump output depends upon engine speed. At low engine speeds, as for example, when the vehicle is descending a grade with the engine being driven by the wheels, the pump output may be insufficient to drive the two hydraulic jacks as rapidly as is called for by the operator's rate of rotating the steering wheel. This can create a dangerous condition, because in such a steering system the speed with which the vehicle can achieve the aspect required for a particular turning radius depends upon how fast the pistons of the steering jacks can move.
Theoretically, it is possible to eliminate the problem by a fluid delivery system in which the pump has a constant output regardless of the speed at which it is driven; but that solution is not feasible because of its complexity and high cost.